App Service Free and Shared (preview) hosting plans are base tiers that run on the same Azure virtual machines as other App Service apps. Some apps might belong to other customers. These tiers are intended to be used only for development and testing purposes.
To use custom domains in Azure App Service, your app's App Service plan must be a paid tier (Shared, Basic, Standard, or Premium). In this step, you make sure that the app is in the supported pricing tier.
To learn how to purchase and set up a custom domain for your App Service web app, see Buy and configure a custom domain name in App Service.